A Cable-Extension Transducer, sometimes also known as a string pot, a draw wire sensor, a string encoder or yo-yo sensor, is a device used to detect and measure linear position and velocity using a flexible cable and spring-loaded spool. Simply mount the transducer's body to a fixed surface and attach the stainless steel cable to the movable object. As the object moves, the transducer produces an electrical signal proportional to the
cable's linear extension or velocity. This signal can then be sent to a PC via a suitable interface.

Inside the transducer's housing, a stainless steel cable is wound on a precisely machined constant diameter cylindrical spool that turns as the measuring cable reels and unreels. To maintain cable tension, a spring is coupled to the spool. The spool is coupled to the shaft of a rotational sensor (an encoder or potentiometer). As the transducer's cable extends along with the movable object, it causes the spool and sensor shafts to rotate. The rotating shaft creates an
electrical signal proportional to the cable's linear extension or velocity. Measuring the movement and displacement of objects is easy with CETs, which can be installed in minutes, fitted into precarious or tight areas and they do not require perfectly parallel alignment. They also offer great flexibility, a small-size to measurement ratio and can be used in a wide variety of applications including industrial factory automation, high-tech medical devices, structural and automotive testing, die-casting or injection
moulding, hydraulic cylinder control or just about anything else you can think of.